1. Field of the Invention
The present invention relates generally to a method and apparatus for refining elemental sulfur and hydrogen gas from gaseous H.sub.2 S. More specifically, the present invention relates to a method and apparatus for vapor phase electrolysis of H.sub.2 S gas using a polysulfide electrolyte disposed in a matrix.
2. Description of the Prior Art
Hydrogen sulfide is present as a contaminant in many valuable fluids. For example, in the petroleum industry, hydrogen sulfide is a commonly found contaminant in production fluids such as oil and gas, and in processing and product streams attendant to the refining of such production fluids. Perhaps the most troublesome occurrence of hydrogen sulfide is its presence in gaseous hydrocarbon mixtures produced from subterranean formations, either separately or concomitantly with liquid petroleum. Such gaseous hydrocarbon mixtures, commonly termed "natural gas", are usually comprised predominantly of methane and ethane with trace amounts of hydrocarbon vapors such as propane, butane, pentane, etc.
Because of its highly corrosive nature, as well as for other reasons, it is usually necessary or at least preferable to remove hydrogen sulfide when it occurs in admixture with hydrocarbons. Also, where a market exists for certain materials of which hydrogen sulfide is a precursor, it usually will be advantageous for economic reasons to obtain, if possible, such materials as by-products of the hydrogen sulfide-hydrocarbon gas separation process.
A process widely accepted for the decomposition of hydrogen sulfide derived from contaminated natural gas wells, or alternately, from the desulfurization of petroleum at oil refineries, is the Claus process. The Claus process involves complete combustion of one third of the hydrogen sulfide to sulfur dioxide: EQU (2H.sub.2 S)+H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O+(2H.sub.2 S)
This is followed by reaction of the SO.sub.2 produced with the remaining H.sub.2 S at 700.degree. F.: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.2H.sub.2 O+3S
The yield of sulfur is about 98.5% efficient and thus a tail-gas clean-up unit is required which substantially increases cost. The minimum size of a Claus unit is about 10 tons of sulfur per day.
Another process under development is termed the Lo Cat process. The Lo Cat process is based on the use of the ferrous/ferric redox couple which oxidizes H.sub.2 S, yielding a sulfur end product. The reduced ferrous species is regenerated to ferric by atmospheric oxygen. While the Claus process is a high temperature gas-phase based process, the Lo Cat process is aqueous based and operates at low temperatures.
Disadvantages of both of these processes involve the resultant end products, since both processes yield only elemental sulfur. The hydrogen that is contained in the hydrogen sulfide is lost either in the form of water or gaseous steam. Thus, the economic efficiency of both the Claus and the Lo Cat process depends solely on the marketability of the sulfur end product.
A number of electrochemical approaches have been proposed for the decomposition of hydrogen sulfide where said decomposition yields both hydrogen gas and elemental sulfur. Almost all of these electrochemical approaches involve the use of aqueous based electrolytes and consist of both "direct" and "indirect" methods of electrochemical decomposition.
In one direct method of electrochemical decomposition, hydrogen sulfide gas is fed directly into the electrochemical cell with the electrolyte where electrochemical reactions take place. These reactions produce hydrogen gas at the cell cathode, and elemental sulfur at the anode. The major difficulty encountered with such direct electrochemical approaches using aqueous electrolytes is that the elemental sulfur generated at the anode "poisons" or deactivates the anode surface, thus precluding further electrochemical reactions from taking place. Further, the elemental sulfur produced in these and similar direct reactions is insoluble in water and thus precipitates over the internal surface of the anolyte chamber of the electrochemical cell.
Generally, in indirect electrochemical approaches a redox couple such as iodide/iodine is employed in the electrochemical cell using an aqueous solvent. The iodine produced as a result of the electrochemical process is subsequently reacted in another reactor vessel with the hydrogen sulfide gas, generating elemental sulfur, protons, and iodide ions. The iodide ions and protons are returned to the electrochemical cell, resulting in the regeneration of iodide as well as the liberation of hydrogen gas. The shuttling of iodide and iodine back and forth between the electrochemical cell and the decomposition reactor is utilized in the breakdown of hydrogen sulfide.
One of the major problems encountered with this and similar indirect approaches is that the sulfur produced in the reactor exists in a sticky plastic form. This form of sulfur must then be dissolved and recrystallized from an organic solvent, e.g., toluene, in order to reduce the sulfur to a soluble form. These additional steps of dissolution and recrystallization are both time and space intensive, thus resulting in an overall loss of system efficiency.